Pressure transducing apparatus is described in German Patent Disclosure Document No. 16 73 938 to which U.S. Pat. No. 3,648,155 corresponds, in which a permanent magnet is positioned in a magnetic circuit which has an air gap and in which is arranged a Hall generator or Hall resistor. The magnetic field lines in the air gap are changed by means of a movable regulating piece which is positioned in dependence on the amplitude of the parameter being measured, for example on the pressure in the intake pipe in the internal combustion engine.